Atomically Thin Population of Colloidal CdSe Nanoplatelets: Growth of Rolled-up Nanosheets and Strong Circular Dichroism Induced by Ligand Exchange

Colloidal two-dimensional cadmium chalcogenides nanoplatelets have recently emerged as a class of semiconductor nanoparticles with the narrowest emission and absorption excitonic bands that are of interest for optical applications. Here, we have developed a synthesis protocol for 2.5-monolayer-(ML) thick CdSe nanosheets as a single population. We found that a two-step synthesis in the presence of water promoted the growth of atomically thin nanosheets with high structural and morphological perfection. Using a seeded-growth technique, we extended the lateral size of nanosheets up to 400 nm, which led to the formation of multiwall rolled-up nanostructures. Ligand exchange of native oleic acid, attached to Cd-rich (001) planes, with achiral thioglycolic acid and chiral N-acetylcysteine retains a scroll-like morphology of nanosheets, in contrast to a thicker 3.5 ML population. A reorientation from the [110] to [100] folding direction was found during the change from an achiral to a chiral ligand. In the case of ligand exchange with chiral N-acetyl-L- or D-cysteine, we demonstrated that 2.5 ML CdSe nanosheets with 400 nm lateral size have circular dichroism with a dissymmetry g factor up to 3 x 10(-3). Strong circular dichroism found for colloidal CdSe nanosheets makes them a promising candidate for polarization-enabled applications, while the growth protocol of the thinnest CdSe nanosheets enriches the known synthesis methods of a set of CdSe nanoplatelet populations.